Demodulation schemes that employ phase locked loops typically do not have an internal signal source that is initially synchronized precisely with a received burst carrier signal. Thus, receivers use the phase locked loops to converge on an incoming frequency and/or phase during the "acquisition" of a received carrier signal. An acquisition phase occurs before valid data can be extracted and includes both carrier acquisition and symbol synchronization.
The steps of carrier acquisition and symbol synchronization are often unproductive overhead time in communication systems. In communication systems conveying lengthy transmissions of large amounts of data, this overhead acquisition phase is relatively unimportant. However, in certain other systems, such as TDMA burst systems, slow overhead acquisition times translate into unacceptably low operating efficiencies.
Various demodulation methods have been used to rapidly extract data from a carrier signal. As an example, the technique known as "differential" demodulation is a robust method of data extraction, but the penalty for using it is a 3-6 dB performance loss that leads to higher transmission power requirements, greater flux density, unsuitable bit error rates, and/or greater likelihood of interference with adjacent channels.
A phase coherent demodulator solves the 3-6 dB penalty of differential demodulation. With coherent demodulation, a phase locked loop accurately achieves carrier synchronization. The problem with the phase locked loop's ability to acquire frequency and phase synchronization is that acquisition times are highly dependent upon the frequency and phase error, or the amount of deviation between the incoming signal and an internal phase locked loop oscillator. Phase locked loops can require an undesirably long time to converge on a signal during acquisition unless the loop filter bandwidth is made very wide. The wider the bandwidth, the more the phase locked loop exhibits phase jitter.
An attempt to overcome this problem by estimating phase based upon a captured block of samples rather than relying solely a phase locked loop has been developed. This technique is called Block Phase Estimation. Block Phase Estimation has one major limitation. It does not work well if the frequency error is more than approximately one percent of the symbol rate. Unfortunately, frequency errors of greater than one percent are not uncommon.
One could simply capture an entire burst and reprocess the burst over and over as taught in U.S. Pat. No 5,440,265 assigned to the assignee of the present invention. But excessive reprocessing of symbols leads to transport delay and should be kept to a minimum. Moreover, reprocessing a block of symbols in a consistently forward order after a phase locked loop has begun its convergence process and has partially acquired a frequency and a phase causes the phase locked loop to experience a discontinuity or transient that extends the carrier acquisition time. In particular, the phase of the incoming signal at the end of the block typically bears no relationship to the phase of the incoming signal at the beginning of the block. Thus, phase convergence efforts of the phase locked loop in a prior block-processing pass are largely lost when a subsequent block-processing pass begins.